Discharge lamps produce light by ionizing a vapor fill material, such as a mixture of rare gases, metal halides and mercury with an electric arc passing between two electrodes. The electrodes and the fill material are sealed within a translucent or transparent discharge vessel that maintains the pressure of the energized fill material and allows the emitted light to pass through it. The ionizable fill material, also known as a “dose,” emits a desired spectral energy distribution in response to being excited by the electric arc. For example, halides provide spectral energy distributions that offer a broad choice of light properties, e.g. color temperatures, color renderings, and luminous efficacies.
High Intensity Discharge (HID) lamps are high-efficiency lamps that can generate large amounts of light from a relatively small source. These lamps are widely used in many applications, including highway and road lighting, lighting of large venues such as sports stadiums, floodlighting of buildings, shops, industrial buildings, and projectors, to name but a few. The term “HID lamp” is used to denote different kinds of lamps. These include mercury vapor lamps, metal halide lamps, and sodium lamps. HID lamps differ from other lamps because their functioning environment requires operation at high temperature and high pressure over a prolonged period of time. Ceramic discharge chambers for HID lamps have been developed to operate at higher temperatures for improved color temperatures, color renderings, and luminous efficacies, while significantly reducing reactions with the fill material. Such lamps with ceramic discharge chambers have been termed “CMH HID” lamps. Metal halide (e.g., CMH) lamps are widely used because they have a higher efficiency than incandescent lamps. This is economically and environmentally beneficial.
Commercially, though, many metal halide lamps contain mercury in their fill. The mercury content in the lamp fill generally does contributes to lamp performance. However, mercury has been considered an environmentally undesirable material. Yet, the problem of replacement or reduction of the mercury content in metal halide lamps is not trivial, since mercury performs so many functions in a metal halide lamp. In general, each function of Hg must be performed by any replacement material (or combination of replacement materials). For example, the mercury functions as (1) a voltage generator, (2) buffer gas, and (3) as a means of reducing I2 formation (for iodide-based lamps). Therefore, in replacing or reducing the amount of mercury, it is necessary to address the problems which arise with regard to these functions of mercury within the metal halide lamp.
Therefore, there is a need for energy-efficient lighting systems which do not contain undesirable amounts of mercury.